Polymeric viscoelastic liquid jets represent key elements of such technologies as textile fiber spinning and electrospinning, as well as in some other applications. Such jets can sustain significant longitudinal stresses of viscoelastic origin. In fiber spinning, aerodynamic jet bending and electrospinning of viscoelastic liquids, both stretching and relaxation are present. The longitudinal stress level is determined by the competition between the stretching and relaxation. Moreover, in the electrically-driven jets in electrospinning the longitudinal viscoelastic stresses begin to build up in the orifice, and become large in the transition zone from the Taylor cone to a completely formed jet. Therefore, viscoelastic jets can possess both significant initial longitudinal viscoelastic stresses generated in the preceding flow domain and charge distribution different than those that create a constant electrical potential everywhere on the surface of the fluid both of which could have dramatic effects on the further evolution of the jet.
In electrospinning of viscoelastic polymer solutions, the length of the initial straight part of the electrified jets is determined by the level of the longitudinal viscoelastic stresses and electric forces. However, little is known about the role of a straight segment with a non-zero initial stress which might arise from the charge distribution. On the contrary, for the free viscoelastic liquid jets rapidly propagating in air and experiencing the aerodynamically-driven bending instability, the crucial role of the initial viscoelastic stresses is well understood theoretically and their level has been measured. In particular, the method of periodic transverse vibrations was proposed which allows measurement of the level of the longitudinal stresses in uncharged jets freely moving in air, as well as an estimate of the viscoelastic relaxation time of the liquids in such jets.
In a typical electrospinning device for the manufacture of nanofibers, a steady state shape of the jet along its path is established when distance between the electrodes is kept short (not more than several centimeters). To create the jet with a steady state shape, the geometry of the orifice, the fluid pressure inside the orifice, a particular viscoelastic solution characterized by its elasticity modulus and relaxation time, a particular electrical potential distribution between the orifice and the collector is established, and any other influence on the jet are held constant. The diameter of the jet can be observed and measured as a function of the position along the path.
Stretching of polymer solutions in electrospun jets begins in the transition zone, between the Taylor cone where liquid is practically unloaded, and the beginning of the thin jet, where the liquid can already be significantly pre-stretched (see FIG. 1). As a result of this pre-stretching, the jet can possess a significant initial stress which might affect its further evolution. The rate of strain in this strong and extremely short pre-stretching process is on the order of 100 s−1 to 1000 s−1 and can be estimated from the results detailed in Schümmer et al., Production of Mono-Dispersed Drops by Forced Disturbance of a Free Jet, Ger. Chem. Eng., 5, pp. 209 to 220 (1982).